iOS/Android: If you do any international calling, you know it can cost a lot of money. There are ways around it, but Keku is a service that assigns your friends abroad numbers that are local to you, so you can call them affordably, even if they're not using the same app. If they are, your calls are completely free.

Keku supports "local" calling in 41 countries, and the service works through its iOS and Android apps. When you call your international friends through Keku, you still have to pay for it (unless they're also using Keku, so it makes sense to try and get them to sign up), but the rates are generally cheaper than your carrier's. To compare rates between the countries you call, check Keku's rates page here.

The service also offers affordable group calling (iOS only, unfortunately), where you just select the participants and Keku handles connecting them all into a conference for you. To give the app a try, grab the app from from the iTunes App Store or Google Play.